As the method for generating a fat distribution image from a cross sectional image data via X ray, a method is known comprising determining a threshold value so as to extract the CT number range from -130 to -100 from the cross sectional image data, based on the finding that the CT number of fat is about -130 to -100.
However, because the fat composition ratio in human bodies varies depending on the tissue, the conventional method described above comprising determining a threshold value has problems in that fat cannot be separated solely from other components in tissues and in that the fat composition ratio in ROI (region of interest) cannot be determined.